Biomass combustion

ABSTRACT

Improved combustion of biomass is achieved by injected first and second streams of biomass from a burner where the first stream of biomass has a median particle size larger than the biomass of the second stream and oxygen is injected with the first stream to provide an oxygen-enriched environment around the larger median sized particles. The oxygen-enriched environment is achieved either by injecting the oxygen directly into the first stream or by premixing the oxygen with the conveying air of the first stream.

CROSS-REFERENCE TO RELATED APPLICATIONS

None.

BACKGROUND

1. Field of the Invention

The present invention relates to biomass burners and methods ofcombusting biomass.

2. Related Art

The emission of carbon dioxide (CO₂) as a cause of global warming is ofcurrent concern to the power industry. The potential role of biomassenergy acquired a new dimension when it was suggested that plantinglarge areas of new forest could slow the increase in atmospheric carbondioxide by removing carbon dioxide from the atmosphere. Therefore, theelectric power industry uses biomass in order to significantly reduceCO₂ emissions.

In a typical staged combustion burner firing biomass, the biomass fuelis combusted with primary combustion air in a first combustion zone. Anynon-combusted fuel is more completely combusted in a second combustionzone downstream of the first combustion zone with secondary combustionair injected around the biomass. If tertiary combustion air is utilized,combustion is completed in a third combustion zone downstream of thesecond combustion zone with tertiary combustion air injected around thesecondary combustion air.

The primary combustion air must have a velocity that is sufficientlyhigh enough to convey the biomass particles from the burner and into thecombustion space. Too low of a primary combustion air velocity willcause the particles to settle within the burner. Conversely, too high ofa primary combustion air velocity will result in a residence time of thebiomass particles that is too short to allow satisfactory burnout of theparticles along the path line through the furnace. This might be noproblem for relatively small biomass particles since they generallyrequire shorter residence times for satisfactory burnout. However, thiscan be a problem for relatively larger biomass particles due to thehigher residence times necessary for their satisfactory burnout alongthe path line.

In order to overcome the above-discussed problem presented by largerbiomass particle sizes, the larger biomass particles produced bycomminuting process can be separated from the smaller particles and thensubjected to the comminuting process again. This milling and re-millingof the particles is complex and costly. It is complex due to the needfor size-separation equipment. Moreover, the milling equipment musteither be over-sized in order to thoroughly process all the biomass feedstock into adequately sized particles or it must necessarily result in alower throughput in terms of adequately sized particles due to thepresence of a side stream of larger particles needing to be recycledback to the milling device. The cost is in part due to the relativelypoor grindability of biomass. While both biomass and coal can be groundto a desired size distribution, the grindability of biomass issignificantly higher than that of coal due to its fibrous nature. Thus,for a given reduction from an initial particle size distribution (suchas d₅₀<1000 mμ) to a final particle size (such as d₅₀<200 mμ), far moreenergy is consumed in achieving the particle size reduction in biomassas compared to coal. Additionally, the amount of energy necessary forsize reduction increases exponentially as the final particle sizedecreases.

Even if the above-described problem associated with larger biomassparticles (i.e., the burnout time exceeds the residence time along thepath line through the furnace) is not experienced when operating aburner at its nominal rated power, it can appear when the burner isoperated at higher powers. The nature of the problem experienced athigher powers is described below.

In comparison to the relatively lower primary combustion air velocitieswhen the burner is operated at lower power, the relatively higherprimary combustion air velocities at higher burner powers changes thepattern of heat transfer from the combusting particles to the furnace.More particularly and in comparison to lower burner powers, relativelyless heat is transferred to portions of the furnace closer to theburners and relatively more heat is transferred to portions of thefurnace relatively distant from the burners. This shift in the amount ofheat transferred to portions of the furnace adjacent the superheater canresult in damage to that portion of the furnace because it is notdesigned for excessive radiative heat transfer.

A related disadvantage is realized for biomass furnaces that wereoriginally commissioned as coal-fired furnaces but which have beenretrofitted for biomass combustion. Coal-fired furnaces are designed tobe heated by a large number of burners. Together, those burners providea nominal power at which the furnace is designed to operate. The nominalpower is related to the heat flux from combustion of the coal to wateror stream in the boiler steam tubes and which is realized in the form ofmechanical or electrical power. If the furnace is retrofitted withconventional biomass burners, at relatively high biomass fuel firingrates the burners may fall well short of the nominal power due tounsatisfactory burnout of the biomass particles. Primarily, this is manybiomass fuels have median particle sizes of 200-500 μm. This may becontrasted with pulverized coal particles which typically have anaverage size of around 60 μm and a d₉₀ of <100 μm. Although the furnacemay have been designed to achieve the nominal power with the morequickly combusting coal particles, the more slowly combusting biomassparticles shifts the pattern of heat transfer from the combustingparticles to the furnace. In particular, less heat is transferred toportions of the furnace adjacent to upstream portions of the path lineand more heat is transferred to portions of the furnace adjacent todownstream portions of the path line. Typical furnaces are not designedfor such a modified heat transfer pattern where much of the heattransfer is shifted downstream along the path line. So, as the flow rateof the biomass fuel from the burner is increased in an attempt toincrease the power, the apparent power of the burner soon reaches alimit beyond which it is difficult to increase by increasing the flowrate of the biomass fuel. As a result of the foregoing, when acoal-fired boiler is retrofitted to combust biomass, more than 20 to 30%reductions in productivity can be experienced.

There have been several biomass combustion processes proposed in thepatent literature.

U.S. patent application Ser. No. 13/479,877, filed May 24, 2012, andentitled “Biomass Combustion”. Application '877 discloses the splittingof an inner flow of biomass (conveyed with air) into a central flow andan annular flow with a splitter in a burner. Oxygen is injected eitherinside the central flow or in an annulus surrounding the central flowand secondary and optionally tertiary air is injected annularly aroundcentral and annular flows of biomass. Due to the difference invelocities between the oxygen flow and the central flow of biomass,enhanced mixing between these two flows results in relatively higheroxygen concentrations surrounding the biomass particles in the centralflow. This tends to allow earlier flame ignition and increases the rateat which the biomass particles combust—all without requiring the biomassto be fired with a high cost, supplementary biofuel such as rapeseedoil. Application '877 does not disclose the use of separate flows ofbiomass fuel having different particle sizes.

U.S. Pat. No. 5,107,777 describes combustion of a low BTU high moisturebiomass such as wood (known as Hog fuel). Biomass is injected into theboiler 15-20 ft above the floor. The combustion air, which is suppliedfrom the bottom of the furnace, is enriched with oxygen to a level ofbetween 0.1 to 7%. Additional oxygen is also injected from the side. Oilburners are fired from the top. It claims that a higher flametemperature is achieved with injection of oxygen.

US 2008/0261161 A1 describes a burner or furnace for the combustion ofbiomass using two or more fuel injection ports located at non-radialinjection angles. The biomass is mixed with oxidizer and then injectedinto the furnace via a cyclonic combustion vortex.

U.S. Pat. No. 6,699,029 B2 describes a boiler system where a low rankfuel is burned to achieve energy generation rate similar to thatachieved with conventional fuels such as coal. It proposes certainoxygen injection methods for reducing the formation of nitrogen oxides(NOx). Operations with typical US-origin coals are described.

Thus, it is an object to provide biomass combustion systems and methodsthat allow complete burnout of relatively larger biomass particles. Itis another object to provide biomass combustion systems and methodscombusting relatively larger biomass particles that do not damageportions of the furnace that are not designed for excessive radiativeheat transfer. It is yet another object to provide biomass combustionsystems and methods combusting relatively larger biomass particles thatdo not limit the apparent power of the burner as the fuel flow rate isincreased.

SUMMARY

There is disclosed a biomass combustion method comprising the followingsteps. A first stream of fuel comprising particulate biomass, air, andoxygen is injected from a burner into a combustion chamber. A secondstream of fuel comprising particulate biomass and air from the burner isinjected into the combustion chamber, the second stream not includingany oxygen apart from the air present in the second stream. The biomass,air and oxygen of the first stream are combusted in the combustionchamber. The biomass and air of the second stream are combusted in thecombustion chamber, wherein the particulate biomass in the first streamhas a median particle size larger than that of the particulate biomassin the second stream.

There is also disclosed a biomass combustion system, comprising abiomass burner, a biomass particle size separator, first and secondbiomass hoppers, first and second blowers, first and second fuelconduits, and a source of oxygen. The biomass particle size separator isadapted and configured to separate a biomass feed stock into first andsecond flows of biomass, the biomass in the first flow having a medianparticle size larger than that of the biomass in the second flow. Thefirst and second biomass hoppers receive the first and second flows ofbiomass, respectively. The first blower is adapted and configured todirect a first stream of biomass from the first biomass hopper, conveyedwith air from the first blower, to the biomass burner. The second bloweris adapted and configured to direct a second stream of biomass from thesecond biomass hopper, conveyed with air from the second blower, to thebiomass burner. The burner comprises a first injector receiving thefirst stream of biomass and a second injector receiving the secondstream of biomass. The first fuel injector receives the first stream ofbiomass and is adapted and configured to inject it from the burner intoa combustion chamber. The second fuel injector receives the secondstream of biomass and is adapted and configured to inject it from theburner into a combustion chamber. The burner receives oxygen from theoxygen source and is adapted and configured to inject it with the firststream of biomass injected from the burner by the first fuel injectoreither premixed with the air of the first stream of biomass or notpremixed with the air of the first stream of biomass.

There is also disclosed a biomass-fired boiler installation, comprisingthe above-disclosed biomass combustion system and a boiler, wherein theburner is oriented to inject the oxygen and first and second streams ofbiomass into a combustion chamber in an interior of the boiler.

The above-disclosed method, system and/or installation can include oneor more of the following aspects:

-   -   the biomass is selected from the group consisting of wood        pellets, straw, hog fuel, crushed olive stones, dried sewage        sludge, wood dust, and combinations thereof.    -   the median particle size of the biomass of the first stream is        less than 300 microns and the median particle size of the        biomass of the second stream is greater than 400 microns.    -   the injected oxygen is no greater than 8% vol/vol of the total        amount of oxidant injected from the burner.    -   the injected oxygen is supplied by an oxygen source selected        from the group consisting of an air separation unit, a vapor        swing adsorption unit, a vaporizer fed with liquefied oxygen, an        oxygen pipeline, and combinations thereof.    -   a biomass feedstock is separated into a first flow of biomass        having a relatively larger median particle size and a second        flow of biomass having a relatively smaller median particle        size, wherein the first stream is derived from the first flow        and the second stream is derived from the second flow.    -   the first flow is fed to a first hopper, the second flow is fed        to a second hopper, the first stream is drawn from the first        hopper, and the second stream is drawn from the second hopper.    -   the oxygen is premixed with the air of the first stream of        biomass.    -   said source of oxygen is selected from group consisting of a        vacuum swing adsorption system, an oxygen pipeline, a cryogenic        air separation unit, and a vaporizer connected to a tank of        liquid oxygen.    -   the oxygen is injected with the first stream of biomass through        injection of the oxygen by an oxygen injector disposed        concentrically within the first fuel injector, wherein the first        and second fuel injectors are annular, and wherein the second        fuel injector is disposed concentrically around the first fuel        injector.    -   the oxygen is injected with the first stream of biomass through        injection of the oxygen by a plurality of oxygen injectors        radially distributed within the first fuel injector, wherein the        first and second fuel injectors are annular.    -   the oxygen is injected with the first stream of biomass through        injection of the oxygen by an oxygen injector disposed within        the first fuel injector, wherein the first fuel injector is        disposed parallel and adjacent to the second fuel injector.    -   there are a plurality of the burners.

BRIEF DESCRIPTION OF THE DRAWINGS

For a further understanding of the nature and objects of the presentinvention, reference should be made to the following detaileddescription, taken in conjunction with the accompanying drawings, inwhich like elements are given the same or analogous reference numbersand wherein:

FIG. 1 is a schematic, cross-sectional view of an embodiment of a burnerfor use with the invention.

FIG. 2 is a schematic, cross-sectional view of another embodiment of aburner for use with the invention.

FIG. 3 is a schematic, cross-sectional view of another embodiment of aburner for use with the invention.

FIG. 4A is an elevation, cross-sectional view of a boiler includingside-fired burners for use with the invention.

FIG. 4B is a top, cross-sectional view of the boiler of FIG. 3A.

FIG. 4C is an elevation, front-face view of an embodiment of one of theside-fired burners of FIGS. 3A and 3B.

FIG. 4D is an elevation, front-face view of another embodiment of one ofthe side-fired burners of FIGS. 3A and 3B.

FIG. 4E is an elevation, front-face view of yet another embodiment ofone of the side-fired burners of FIGS. 3A and 3B.

DESCRIPTION OF PREFERRED EMBODIMENTS

By injecting oxygen with a stream of relatively larger sized biomassparticles conveyed with air (i.e., the first stream), the resultantoxygen-enriched air surrounding the particles allows them to be ignitedsooner. Satisfactory burnout is also achieved. A separate stream ofrelatively smaller sized biomass particles (i.e., the second stream)need only be combusted with air (no oxygen need be injected with thestream) because their smaller size already allows earlier ignition andshorter burnout times. Because the combustion air is not globallyenriched with oxygen, the burner consumes far less oxygen. Becausesatisfactory burnout of the larger size biomass particles may beachieved, it is not necessary to expend significant amounts ofadditional energy in reducing the size of the larger size biomassparticles in order to obtain the small size particles that enjoy earlierignition and shorter burnout times. The oxygen may be injected with therelatively larger biomass particles by premixing the oxygen with the airconveying the relatively larger biomass particles or the oxygen may beinjected into the stream of larger biomass particles from an oxygeninjector inside the burner.

A stable flame rooted near the face of the burner is achieved becausethe locally high concentration of oxygen in the stream of larger biomassparticles allows those particles to be ignited more easily and at anearlier point than they would otherwise without such a locally highconcentration of oxygen. Thus, a combustion reaction between oxygen(from the oxygen and the air) and the larger biomass particles iscommenced at an upstream zone adjacent the burner face. The greatersurface area to volume or mass ratio of the relatively smaller biomassparticles allows them to be combusted with only air (and not oxygen).Thus, there is no need to globally enrich the air injected from theburner from all sources of combustion air.

The type of burner is not critical to the invention.

A pipe-in-pipe burner may be utilized where one of the two streams ofbiomass is injected from the inner pipe while the other of the twostreams of biomass is injected from the outer pipe. In this case, theoxygen is injected in one of a variety of ways. It may be injected in apremixed state with the air of the stream of larger biomass particles.It may instead be injected from an oxygen injector disposed within theinner pipe in which case the stream of larger biomass particles isinjected from the inner pipe. It may instead be injected from aplurality of oxygen injectors radially distributed within the outer pipein which case the stream of larger biomass particles is injected fromthe outer pipe.

The burner may instead be of the side wall fired type including two fuelports (one for each stream of biomass) and an overfire air port. Theoxygen may be injected in a premixed state with the air of the stream oflarger biomass particles. It may instead by injected from an oxygeninjector disposed within the fuel port from which the stream of largerbiomass particles is injected. In either case, the fuel ports and/oroxygen injector may be of any configuration.

The combustion air may be swirled or not. The combustion air may be twostreams each one of which conveys one of the streams of biomass oradditional combustion air streams may be utilized including a secondarycombustion air stream surrounding or adjacent to or spaced from thebiomass streams and even a tertiary combustion air stream surrounding oradjacent to or spaced from the secondary combustion air stream. Eitheror both of the secondary and tertiary combustion air streams may beswirled.

While the biomass fuel may be any biomass fuel known in the art,typically it is wood pellets, straw, or so-called hog fuel.

The oxygen is industrially pure oxygen. The specific purity of theindustrially pure oxygen depends upon the method of production andwhether or not the produced oxygen is further purified. For example, theindustrially pure oxygen may be gaseous oxygen from an air separationunit that cryogenically separates air gases into predominantly oxygenand nitrogen streams in which case the gaseous oxygen has aconcentration exceeding 99% vol/vol. The industrially pure oxygen may beproduced through vaporization of liquid oxygen (which was liquefied fromoxygen from an air separation unit, in which case it, too, has a purityexceeding 99% vol/vol. The industrially pure oxygen may be also beproduced by a vacuum swing adsorption (VSA) unit in which case ittypically has a purity of about 92-93% vol/vol. The industrially pureoxygen may be sourced from any other type of oxygen productiontechnology used in the industrial gas business.

As best illustrated in FIG. 1, a first type of pipe-in-pipe burnerincludes an inner first fuel injector 1, an outer second fuel injector12, and a secondary combustion air pipe 14 disposed within a burnerblock B which is mounted on a wall W of the combustion space C. Theburner injects a first stream 8 of larger particle size biomass conveyedwith air from the fuel injector 1. Oxygen is injected into stream 8 froma single oxygen injector 2 which is disposed within the first fuelinjector 1. A second stream 6 of smaller particle size biomass conveyedwith air is injected from the second fuel injector 12. Finally, a stream5 of secondary combustion air is injected from the secondary air pipe 14through swirler 10. The biomass from the first stream 8 combusts withthe oxygen and conveying air in an inner core of the flame while thebiomass from the second stream 6 combusts with conveying air in an outerportion of the flame.

As best shown in FIG. 2, a second type of pipe-in-pipe burner againincludes an inner first fuel injector 1, an outer second fuel injector12, and a secondary combustion air pipe 14 disposed within a burnerblock B which is mounted on a wall W of the combustion space C. Theburner injects a first stream 8 of smaller particle size biomassconveyed with air from the fuel injector 1. A second stream 6 of smallerparticle size biomass conveyed with air is injected from the second fuelinjector 12. Instead of injecting oxygen into one of the streams 8, 6,the oxygen is already premixed with the conveying air of one of thestreams 8, 6, so there are no oxygen injectors per se within the burner.Finally, a stream 5 of secondary combustion air is injected from thesecondary air pipe 14 through swirler 10. If the larger size biomass isinjected as stream 8, it combusts with oxygen-enriched conveying air inan inner core of the flame while the biomass from the second stream 6combusts with the conveying air in an outer portion of the flame. If thelarger size biomass is injected as stream 6, it combusts withoxygen-enriched conveying air in an outer portion of the flame while thebiomass from the first stream 8 combusts with the conveying air in theinner core of the flame.

In a variation of the burner of FIG. 1 and as best illustrated in FIG.3, a third type of pipe-in-pipe burner includes an inner first fuelinjector 1, an outer second fuel injector 12, and a secondary combustionair pipe 14 disposed within a burner block B which is mounted on a wallW of the combustion space C. The burner injects a first stream 8 ofsmaller particle size biomass conveyed with air from the fuel injector1. A second stream 6 of smaller particle size biomass conveyed with airis injected from the second fuel injector 12. Oxygen is injected intostream 8 from a plurality of oxygen injectors 2 which is radiallydistributed within the second fuel injector 12. There are at least threeoxygen injectors 2 in the burner of FIG. 3 with an upper number onlybeing limited by the available space and the pressure drop in the stream6 created by the injectors 2. Finally, a stream 5 of secondarycombustion air is injected from the secondary air pipe 14 throughswirler 10. The biomass from the first stream 8 combusts with conveyingair in an inner core of the flame while the biomass from the secondstream 6 combusts with the oxygen and conveying air in an outer portionof the flame.

The oxygen injector 2 may be configured in any one of several ways knownin the field of oxy-combustion. Thus, the oxygen lance 2 may be astraight tube with a constant diameter. Alternatively, the oxygen lance2 may diverge at its outlet end where the oxygen mixes with the one ofstreams 6, 8.

As best shown in FIGS. 4A-E, the burner may instead be configured as aside wall-fired burner 32 mounted on one of the walls of the interior ofa boiler 31.

FIG. 4C illustrates a first variation of the burner 32 where the firststream of larger size biomass particles is injected from a first fuelport 42 and the second stream of smaller size biomass particles isinjected from a second fuel port 41. Secondary combustion air isinjected from air ports 44 above and below the fuel ports 41, 42. Theoxygen is injected from an oxygen channel (i.e., oxygen injector) 43disposed vertically within the first fuel port 42. FIG. 4D illustrates asecond variation of the burner 32 similar to that of FIG. 4E where theoxygen channel (i.e., oxygen injector) 43 is disposed horizontallywithin the first fuel port 42. Finally, FIG. 4E illustrates a thirdvariation of the burner 32 similar to those of FIGS. 4D-E where theoxygen channel (i.e., oxygen injector) 43 has a cross-shape. In each ofthe burners of FIGS. 4C-E, the surface area of the oxygen injectortypically does not take over 30% of the surface area of the first fuelport 42 from which the larger size biomass stream is injected. Insteadof the oxygen channel (i.e., oxygen injector) 43 configurations of FIGS.4C-E, the oxygen injector could comprise a plurality of pipes adjacentto one another instead of a channel (not shown).

Regardless of whether the oxygen is injected according to the burnerconfigurations of FIG. 1, 2, 3, or 4C-E, the presence of oxygen in astream of larger size biomass (whether the oxygen is pre-mixed with thecombustion air or injected into such stream) helps increase burnout ofthat fuel in comparison to a conventional biomass burner where no suchoxygen injection is employed. Burnout is increased because the localoxygen concentration surrounding the larger biomass particles isincreased. An oxygen-enriched atmosphere at this region not only startscombustion of volatile components in the biomass particles earlier butalso starts combustion of char earlier. As a result, satisfactoryburnout of the biomass particles is completed in the path line of thebiomass particles inside the furnace at a point earlier in comparison tobiomass particles from biomass burners where no such oxygen injection isperformed. There is no need to inject oxygen into the stream of smallerbiomass particles or enrich its conveying air because the smaller sizeof those particles allows them to be satisfactory burned out morequickly.

Faster burnout of the biomass particles is advantageous for allowingsatisfactory operation of the biomass burner at higher apparent powers.This will be clearly evident when compared to operation of aconventional biomass burner in which no oxygen injection or premixing ofthe conveying air is performed. When conventional biomass burners areoperated at lower powers, the flow rate of primary combustion airnecessary for satisfactory conveyance of the biomass particles has avelocity sufficiently low that satisfactory burnout of the biomassparticles may be achieved over the path line traveled by the particlesthrough the furnace. At higher burner powers, the flow rate of primarycombustion air that is necessary for satisfactory conveyance of thebiomass particles must be increased because the total mass of solidbiomass particles is increased. As the flow rate of the primarycombustion air is increased, it will soon reach a velocity that is toohigh to allow satisfactory burnout of the solid biomass particles alongthe path line through the furnace and enter the superheater. In otherwords, the residence time of a combusting biomass particle is decreasedwhen higher velocity combustion air is used (such as at higher burnerpowers). Such a situation creates several disadvantages.

One disadvantage is related to wear to the furnace. In comparison to therelatively lower combustion air velocities when the burner is operatedat lower power, the relatively higher combustion air velocities athigher burner powers changes the pattern of heat transfer from thecombusting particles to the furnace. More particularly and in comparisonto lower burner powers, relatively less heat is transferred to portionsof the furnace closer to the burners and relatively more heat istransferred to portions of the furnace relatively distant from theburners. This shift in the amount of heat transferred to portions of thefurnace adjacent the superheater can result in damage to that portion ofthe furnace because it is not designed for excessive radiative heattransfer.

The second disadvantage is realized for biomass furnaces that wereoriginally commissioned as coal-fired furnaces but which have beenretrofitted for biomass combustion. Coal-fired furnaces are designed tobe heated by a large number of burners. Together, those burners providea nominal power at which the furnace is designed to operate. The nominalpower is related to the heat flux from combustion of the coal to wateror stream in the boiler steam tubes and which is realized in the form ofmechanical or electrical power. If the furnace is retrofitted withconventional biomass burners, at relatively high biomass fuel firingrates the burners may fall well short of the nominal power due tounsatisfactory burnout of the larger biomass particles. Primarily, thisis because the larger biomass particles (with a median size of around200-500 μm) combust more slowly than typical pulverized coal particles(with an average size of around 60 μm). Although the furnace may havebeen designed to achieve the nominal power with the more quicklycombusting coal particles, the more slowly combusting larger biomassparticles shifts the pattern of heat transfer from the combustingparticles to the furnace. In particular, less heat is transferred toportions of the furnace adjacent to upstream portions of the path lineand more heat is transferred to portions of the furnace adjacent todownstream portions of the path line. Typical furnaces are not designedfor such a modified heat transfer pattern where much of the heattransfer is shifted downstream along the path line. So, as the flow rateof the biomass fuel from the burner is increased in an attempt toincrease the power, the apparent power of the burner soon reaches alimit beyond which it is difficult to increase by increasing the flowrate of the biomass fuel.

When conventional biomass burners are instead fed only with relativelysmaller size biomass particles in an effort to avoid the above twodisadvantages, such an avoidance still results in a higher costassociated with more grinding/milling of all of the biomass and/orwasting of the larger biomass particles after separation from thesmaller biomass particles. The energy consumed in milling/grindingbiomass is a major component of its cost as a fuel.

In contrast, by enriching the larger biomass particles with oxygen(either through injection or premixing with the conveying of thatstream), the above disadvantages may be avoided without having toincrease the energy consumption and cost associated with conventionalbiomass burner operation. The higher oxygen concentrations surroundingthe larger biomass particles tends to ignite the flame earlier andincreases the rate at which the larger biomass particles combust. As aresult, the impact of the downstream shift in heat transfer that wouldotherwise be experienced in furnaces fired with conventional biomassburners (fired with larger size biomass particles or a mixture of largerand smaller biomass particles) is reduced or nullified by the increasein the rate of combustion of the biomass particles afforded by thelocalized oxygen-enriched environment. Because the distribution of heattransfer from the combusting particles to the furnace more closelymatches the distribution of heat transfer that the furnace wasoriginally designed for when it was commissioned as a coal-firedfurnace, the apparent power of the burner may still be increased throughan increase in the flow rate of the biomass fuel from the burner. Also,the above-described increase in furnace wear caused by conventionalbiomass burners is either decreased or avoided.

Thus, the invention provides multiple benefits. The invention canimprove the overall system efficiency with minimum modifications on thecurrent boiler combustion system. It can reduce a power plant's CO₂ footprint. Oxygen enrichment will reduce the flue gas volume. Oxygenenrichment of only the stream of larger biomass particles is less costlythan global enrichment of the burner's combustion air. The avoidance of,or reduction in use of, a higher heating value auxiliary fossil fuel orbiomass fuel reduces the operational cost. Finally, the apparent burnerpower may be increased beyond levels achievable with conventionalbiomass burners. Excess furnace wear may be reduced or avoided.

Preferred processes and apparatus for practicing the present inventionhave been described. It will be understood and readily apparent to theskilled artisan that many changes and modifications may be made to theabove-described embodiments without departing from the spirit and thescope of the present invention. The foregoing is illustrative only andthat other embodiments of the integrated processes and apparatus may beemployed without departing from the true scope of the invention definedin the following claims.

What is claimed is:
 1. A biomass combustion method, comprising the stepsof: injecting a first stream of fuel comprising particulate biomass,air, and oxygen from a burner into a combustion chamber; injecting asecond stream of fuel comprising particulate biomass and air from theburner into the combustion chamber, the second stream not including anyoxygen apart from the air present in the second stream; combusting thebiomass, air and oxygen of the first stream in the combustion chamber;and combusting the biomass and air of the second stream in thecombustion chamber, wherein the particulate biomass in the first streamhas a median particle size larger than that of the particulate biomassin the second stream.
 2. The biomass combustion method of claim 1,wherein the biomass is selected from the group consisting of woodpellets, straw, hog fuel, crushed olive stones, dried sewage sludge,wood dust, and combinations thereof.
 3. The biomass combustion method ofclaim 1, wherein the median particle size of the biomass of the firststream is less than 300 microns and the median particle size of thebiomass of the second stream is greater than 400 microns.
 4. The biomasscombustion method of claim 1, wherein the injected oxygen is no greaterthan 8% vol/vol of the total amount of oxidant injected from the burner.5. The biomass combustion method of claim 1, wherein the injected oxygenis supplied by an oxygen source selected from the group consisting of anair separation unit, a vapor swing adsorption unit, a vaporizer fed withliquefied oxygen, an oxygen pipeline, and combinations thereof.
 6. Thebiomass combustion method of claim 1, further comprising the step ofseparating a biomass feedstock into a first flow of biomass having arelatively larger median particle size and a second flow of biomasshaving a relatively smaller median particle size, wherein the firststream is derived from the first flow and the second stream is derivedfrom the second flow.
 7. The biomass combustion method of claim 6,wherein: the first flow is fed to a first hopper; the second flow is fedto a second hopper; the first stream is drawn from the first hopper; andthe second stream is drawn from the second hopper.
 8. The biomasscombustion method of claim 6, wherein the oxygen is premixed with theair of the first stream of biomass.
 9. A biomass combustion system,comprising a biomass burner, a biomass particle size separator, firstand second biomass hoppers, first and second blowers, first and secondfuel conduits, and a source of oxygen, wherein: the biomass particlesize separator is adapted and configured to separate a biomass feedstock into first and second flows of biomass, the biomass in the firstflow having a median particle size larger than that of the biomass inthe second flow; the first and second biomass hoppers receive the firstand second flows of biomass, respectively; the first blower is adaptedand configured to direct a first stream of biomass from the firstbiomass hopper, conveyed with air from the first blower, to the biomassburner; the second blower is adapted and configured to direct a secondstream of biomass from the second biomass hopper, conveyed with air fromthe second blower, to the biomass burner; the burner comprises a firstinjector receiving the first stream of biomass and a second injectorreceiving the second stream of biomass; the first fuel injector receivesthe first stream of biomass and is adapted and configured to inject itfrom the burner into a combustion chamber; the second fuel injectorreceives the second stream of biomass and is adapted and configured toinject it from the burner into a combustion chamber; the burner receivesoxygen from the oxygen source and is adapted and configured to inject itwith the first stream of biomass injected from the burner by the firstfuel injector either premixed with the air of the first stream ofbiomass or not premixed with the air of the first stream of biomass. 10.The biomass combustion system of claim 9, wherein said source of oxygenis selected from group consisting of a vacuum swing adsorption system,an oxygen pipeline, a cryogenic air separation unit, and a vaporizerconnected to a tank of liquid oxygen.
 11. The biomass combustion systemof claim 9, wherein: the oxygen is injected with the first stream ofbiomass through injection of the oxygen by an oxygen injector disposedconcentrically within the first fuel injector; the first and second fuelinjectors are annular; and the second fuel injector is disposedconcentrically around the first fuel injector.
 12. The biomasscombustion system of claim 9, wherein: the oxygen is injected with thefirst stream of biomass through injection of the oxygen by a pluralityof oxygen injectors radially distributed within the first fuel injector;and the first and second fuel injectors are annular.
 13. The biomasscombustion system of claim 9, wherein: the oxygen is injected with thefirst stream of biomass through injection of the oxygen by an oxygeninjector disposed within the first fuel injector; and the first fuelinjector is disposed parallel and adjacent to the second fuel injector.14. A biomass-fired boiler installation, comprising the biomasscombustion system of claim 9 and a boiler, wherein the burner isoriented to inject the oxygen and first and second streams of biomassinto a combustion chamber in an interior of the boiler.
 15. Thebiomass-fired boiler installation of claim 14, wherein there are aplurality of the burners.